Synthesis of a novel ZnFe2O4/porous biochar magnetic composite for Th(IV) adsorption in aqueous solutions

A novel magnetic zinc ferrite/porous biochar composite (c-PBC/ZF) was prepared and utilized to remove radioactive thorium (Th) (IV) from aqueous solutions. Zinc chloride (ZnCl2) not only activates biochar during the hydrothermal synthesis of c-PBC/ZF but also serves as a basic raw material for the synthesis of zinc ferrite. In addition, nitrilotriacetic acid (NTA) plays a dual key role in catalyzing and generating large numbers of carboxyl groups (–COOH) during synthesis. The characterization results confirmed that ZnFe2O4 (ZF) particles with cubic spinel structures were successfully embedded into the porous biochar (PBC) matrix, which endowed the composite with superparamagnetism and enabled higher Th(IV) adsorption performance than that of single PBC and ZF nanoparticles. At room temperature, batch experiments showed that the removal rate of c-PBC/ZF for Th(IV) was 97.95% when the compound ratio of c-PBC/ZF was 1:1, the pH value was 4 and the adsorbent dosage was 1.2 g/L. Through the analysis of adsorption kinetics, isotherms and thermodynamics, the Th(IV) adsorption process is in good agreement with the experimental data of the quasi-second-order kinetics model and Langmuir model, indicating that the process is a spontaneous, endothermic and monolayer adsorption process. FTIR analysis verified that the main mechanism of Th(IV) removal by c-PBC/ZF was Th–O and Th(IV) complexation with a carboxyl group and a hydroxyl group. Finally, the desorption and regeneration study proved that c-PBC/ZF has a highly efficient cycling performance as an adsorbent for recovering Th(IV) from wastewater. Therefore, c-PBC/ZF is a promising adsorbent for recovering Th(IV) from wastewater.